The Median Tectonic Line is Japan's longest onshore active fault, running about 1,000 kilometres from Kyushu through Shikoku and across the Kii Peninsula into central Honshu. It began as an ancient geological suture between two contrasting rock belts and has been reactivated as an active fault in its western portion.
Today its most active stretches behave as a right-lateral strike-slip fault, driven by the oblique subduction of the Philippine Sea Plate beneath southwest Japan along the Nankai Trough. Slip rates on the fastest segments reach several millimetres per year, and the line passes close to cities such as Matsuyama and Tokushima on Shikoku.
The fault is capable of large inland earthquakes: paleoseismic studies show repeated ruptures of magnitude 7 or greater, and Japanese hazard agencies rank several of its segments among the most likely sources of a future major inland quake in the region.